1. Field of the Invention
The present invention relates to a so-called spin-valve type thin film element in which electric resistance varies with the relation between magnetization direction of a pinned magnetic layer and magnetization direction of a free magnetic layer being affected with the external magnetic field, especially to a spin-valve type thin film element formed so as to be able to obtain a favorable asymmetry.
2. Description of the Related Art
FIG. 8 is a cross section close to the ABS surface (Air Bearing Surface) of a spin-valve type thin film element (spin-valve type tin film magnetic head) for sensing recording magnetic field from a recording medium such as a hard disk.
An anti-ferromagnetic layer 30, a pinned magnetic layer 2, a non-magnetic conductive layer 3 and a free magnetic layer 31 are layered in this spin-valve type thin film element, bias layers 5, 5 being formed on both side.
Usually, a Fe--Mn (iron--manganese) alloy film or Ni--Mn (nickel--manganese) alloy layer is used for the anti-ferromagnetic layer 30, a Ni--Fe (nickel--iron) alloy layer is used for the pinned magnetic layer 2 and free magnetic layer 31, a Cu (copper) film is used for the non--magnetic conductive layer 3 and a Co--Pt (cobalt--platinum) alloy layer is used for the bias layers 5, 5. The reference numerals 6 and 7 refer to an underlaying layer and protective layer formed of a non-magnetic material such as Ta (tantalum).
As shown in the figure, the anti-ferromagnetic layer 30 and pinned magnetic layer 2 are formed in direct contact with each other and the pinned magnetic layer 2 are put into a single magnetic domain state along the Y-direction (the height direction) by an exchange anisotropic magnetic field arising from an exchange coupling at the interface between the pinned magnetic layer and anti-ferromagnetic layer 30, the magnetization direction being pinned along the Y-direction. The exchange anisotropic magnetic field is generated at the interface between the anti-ferromagnetic layer 30 and pinned magnetic layer 2 by applying an annealing treatment (a heat treatment) while a magnetic field is applied along the Y-direction when the anti-ferromagnetic layer 30 is composed of a Ni--Mn alloy film. When the anti-ferromagnetic layer 30 is composed of a Fe--Mn alloy film, on the other hand, the exchange anisotropic magnetic field is generated at the interface between the anti-ferromagnetic layer 30 and pinned magnetic layer 2 by forming the film in a magnetic field.
The magnetization direction of the free magnetic layer 31 is aligned along the X-direction under the influence of the hard bias layers 5, 5 magnetized along the X-direction (the track direction).
In the method for producing the spin-valve type thin film element, six layers from the underlayer 6 to the protective layer 7 are at first formed and the side face of the foregoing six layers is shaved off in the following etching step such as an ion-milling process to form inclined faces, followed by forming the hard bias layers 5, 5 on both side of the six layers.
A static current (sensing current) is imparted in this spin--valve type thin film element from conductive layers 8, 8 to the pinned magnetic layer 2, non-magnetic conductive layer 3 and free magnetic layer 31. While the scanning direction of the recording medium such as the hard disk is Z-direction, magnetization of the free magnetic layer 4 turns from X-direction to Y-direction when a leakage magnetic field from the recording medium is applied along the Y-direction. Electric resistance is varied in response to the relation between the variation of the magnetization direction in this free magnetic layer 31 and the pinned magnetization direction of the pinned magnetic layer 2, and the leakage magnetic field from the recording medium is sensed by the voltage changes based on the variation of this electric resistance level.
Only the ABS surface (the front surface) is exposed to outside and the other surfaces are covered with insulation films of, for example, Al.sub.2 O.sub.3 in the spin-valve type thin film element shown in FIG. 8. Since the spin-valve type thin film element has a multi-layer structure of metallic films, the thermal expansion coefficient of the spin-valve type thin film element is made to be larger than the thermal expansion coefficient of the insulation film covering the spin-valve type thin film element. Therefore, a tensile stress along the Y-direction (the height direction) shown in the drawing is applied on the spin-valve type thin film element.
When magnetostriction of the free magnetic layer 31 constituting the spin-valve type thin layer element assumes a positive value in the state described above, magnetization of the free magnetic layer 31 is induced by being inclined to the Y-direction shown in the drawing owing to a reverse magnetostrictive effect.
As hitherto described, magnetization of the free magnetic layer 31 is aligned along the X-direction (the direction of track width) shown in the drawing owing to the free bias layers 5, 5 and magnetization of the free magnetic layer 31 is fluctuated by the leakage magnetic field from the recording medium, thus sensing the leakage magnetic field from the recording medium.
However, when a tensile stress along the Y-direction shown in the drawing is applied to the spin-valve type thin film element and magnetostriction of the free magnetic layer 31 assumes a positive value, the magnetization is not favorably reversed against the leakage magnetic field from the recording medium because magnetization of the free magnetic layer is induced by being inclined to the Y-direction shown in the drawing, so that horizontal symmetry of the regenerative output waveform is deformed.
The horizontal non-symmetry of the regenerative output waveform is referred to asymmetry, which is represented by &lt;[.DELTA.R(-H(Oe))-.DELTA.R(+H(Oe))]/[.DELTA.R(-H (Oe))+.DELTA.R(+H(Oe))].times.100&gt; (R represents resistance), wherein the leakage magnetic field from the recording medium to be applied along the Y-direction shown in the drawing is represented by .+-.H (Oe; Oerstead). .DELTA.R(-H(Oe) above means the variation of resistance AR when the leakage magnetic field is -H.
The closer the asymmetry as described above to zero is, the higher becomes the horizontal symmetry of the regenerative output waveform, improving regenerative characteristics.
The inventors of the present invention measured asymmetry of the spin-valve type thin film element when a tensile stress is applied along the Y-direction of the free magnetic layer 31 shown in the drawing along with imposing an additional positive magnetostriction on the free magnetic layer 31.
A uniform magnetostriction of +2.times.10.sup.-6 was applied to the free magnetic layer 31 while giving tensile stress of 0 MPa, 15 MPa, 70 MPa and 192 MPa along the Y-direction of the spin-valve type thin film element shown in the drawing, thereby investigating the relation between the sensing current Is and asymmetry (%) against respective tensile stresses. The experimental results are shown in FIG. 9.
It is clear from the figure that the larger the tensile stress is, the more asymmetry is deteriorated (departing from zero).
The sensing current dependent slopes of asymmetry by applying respective tensile stresses indicate that, the larger the stress is, the steeper becomes the slope of asymmetry.
Although asymmetry approaches to approximately zero at a sensing current of about 8 mA when a stress of 192 MPa where the slope of the sensing current dependence of asymmetry is largest is applied, asymmetry is radically deteriorated (becomes larger than zero) when the sensing current reaches to 8 mA or more because the slope is so steep.
It is difficult as hitherto described to make asymmetry to approach to zero so long as a large tensile stress is applied along the Y-direction when magnetostriction of the free magnetic layer 31 assumes a positive value. It should be noted that a tensile stress of 200 to 300 MPa is usually applied along the Y-direction shown in the drawing in the spin-valve thin film element, thereby asymmetry is practically thought to be more deteriorated than the experimental result shown in FIG. 9.